The thermoresponsiveness of ventromedial hypothalamic (VMH) neurons following repeated scrotal thermal stimulation of rats maintained at normothermic or acutely hypothermic core temperatures.

Neurons within the central nervous system, including those within the ventromedial hypothalamic (VMH) nucleus, alter their neuronal activity in response to scrotal thermal stimulation. This study set out to establish if the thermoresponsiveness of VMH neurons becomes modified to repeated trials of scrotal thermal stimulation. VMH extracellular activity was recorded with a glass micropipette filled with 0.5 M sodium acetate in urethane-anaesthetized male Sprague-Dawley rats following 20 min scrotal heating (scrotal packs, 2 x 2 cm, filled with 40 degrees C hot water) and scrotal cooling (scrotal packs filled with ice). In study A, VMH neurons were tested to 2 trials of scrotal heating and cooling with colonic temperatures (T(c)s) servo-controlled at 37 degrees C during both trials. In study B, VMH neurons were tested for 3 trials of scrotal thermal stimulation, with T(c)s servo-controlled at 37, 35 and 33 degrees C during the 3 trials. In study A, 42 VMH neurons were isolated. Based on their thermal coefficients (TCs) to the 1st trial of scrotal heating and cooling, 12 VMH neurons were classified as warm-responsive neurons (WRNs), 10 as cold-responsive neurons (CRNs) and 20 were temperature-non-responsive neurons (TNRNs). Of the neurons recorded long enough to test their thermoresponsiveness to 2nd trial of scrotal thermal stimulation (9 out of 12 WRNs, 7 out of 10 CRNs and 18 out of 20 TNRNs), the mean TCs of each type of VMH neuron did not significantly change between the 2 trials. In study B, 65 VMH neurons were isolated and 11 out of 22 WRNs, 7 out of 13 CRNs and 15 out of 30 TNRNs had their thermoresponsiveness tested to 3 trials of scrotal heating and cooling, with T(c)s kept at 37, 35 and 33 degrees C, respectively, for these trials. The mean TCs for VMH WRNs, CRNs and TNRNs again did not change between the 3 trials of scrotal thermal stimulation. However, mean basal firing rates did increase significantly of all recorded VMH neurons between the 1st and 3rd trials of scrotal heating and cooling as T(c)s were acutely lowered from 37 to 33 degrees C. Results demonstrated that VMH thermoresponsive neurons retain their responsiveness to repeated trials of scrotal heating and cooling of animals maintained at normothermic (37 degrees C) or acutely hypothermic (35 and 33 degrees C) temperatures.